1. Field of the Invention
The present invention relates to a toner used as a developer for developing an electrostatic charge image formed by electrophotography, electrostatic recording, etc., and an image forming apparatus using the toner.
2. Description of the Related Art
In recent years, image forming apparatuses have been required to produce high-quality images, and studies have been made to develop a toner excellent in heat resistant storage stability, transfer property, flowability, filming resistance, charge property, etc.
In order to improve heat resistant storage stability of a toner, there is proposed a toner having a core-shell structure in which a shell layer containing a resin different from a binder resin is formed on the surface of the toner particles (see, e.g., Japanese Patent Application Laid-Open (JP-A) No. 2006-267950). However, the problem of this proposed toner is that a pigment cannot be dispersed throughout the shell layer but is lopsidedly deposited on the surface, which degrades the transfer property and flowability of the toner to degrade the image property.
In order to improve transfer property and flowability of a toner, there are proposed toners in which an external additive containing inorganic particles is added (see, e.g., JP-A Nos. 2005-173480 and 2010-128216). These proposals teach to impart a spacer effect to the surface of the toner particles and suppress adhesion between toner particles and occurrence of aggregates of toner particles while the toner is conveyed to thereby improve the transfer property and flowability of the toner and reduce fault images due to degradation of the toner. However, the problem of these proposals is that the toner includes the inorganic particles in an excessive amount, and the inorganic particles tend to be detached. The detached inorganic particles accelerate wear of the cleaning blade or cause filming to thereby degrade the charge property and produce fault images.
In order to improve filming resistance and charge property of a toner, there are proposed toners in which silica having a relatively broad particle size distribution is added as an external additive (see, e.g., JP-A Nos. 11-174731 and 2010-243664). These proposals explain that use of an external additive having a relatively broad particle size distribution allows the toner to have a broad range of charge properties according to the particle size distribution thereof. Silica is greater than alumina in the ability to electrically charge the toner. Silica is not limited to silica derived by sol-gel method (sol-gel silica), and use of dry silica is encouraged because dry silica can impart a broad range of charge properties to the toner according to the particle size distribution of the toner. However, the problem of these proposed toners is that their heat resistant storage stability is poor because they are toners using only dry silica, and that their charge property is significantly degraded under high-temperature, high-humidity conditions.
Therefore, there is a demand for a toner that can secure at the same time, heat resistant storage stability, transfer property, flowability, and filming property, and charge stability even under high-temperature, high-humidity conditions, and has excellent low temperature fixability in terms of energy saving.